Sun are finally introducing their new range of Opteron boxes. That'll certainly keep them with credible systems for most of the market, and given how quiet things have been at the high-end (Ultrasparc has been canned, the story was that they'd go with Fujitsu SPARC64, but nobody is saying much publicly), I wouldn't be surprised if Sun quietly withdrew from the big iron to concentrate on the mid-range and below.
The dual-core Opteron systems seem to manage to put out a respectable enough transaction rate to make even a quad socket system look quite a decent database server. HP are already strong in this area, with benchmarks comparing a quad 2.6GHz Opteron single core (CPU model 852) with a quad 2.2GHz Opteron dual core (CPU model 875), with the dual core system coming in 72.5% faster. Admittedly this was helped by more memory and disks, but it'd be a poor CPU comparison if you let the system bottleneck be IO, and it'd hurt the price/performance to give the slower system more disk than it could use. I noticed that the quad 875 comes in 21% faster than the 32 way Alpha 21264 (Compaq GS320 Wildfire), which was one of the then top-end systems I ran benchmarks on, four years ago now.
Interesting comparison from Unisys on TPC-C running RHEL 4.0 and SuSE Enterprise 9 on identical hardware (16 way Itanium), both with Oracle 10g. There's just a 1.6% difference in transaction rate, with Redhat ahead. So, there's no real performance benefit from the available choices for enterprise Linux.